Various processes for producing thermo-expansive microcapsules, wherein thermoplastic polymer is used for micro-encapsulating a volatile expanding agent having a gasification point lower than the softening point of said polymer, have been studied. An overall process for producing thermo-expansive microcapsules is described in Japanese Patent Publication No. Sho 42-26524, and a process for producing thermo-expansive microcapsules having polymeric shell wall of uniform sickness is described in U.S. Pat. No. 3,615,972.
Although those processes can produce thermo-expansive microcapsules, they cannot produce thermo-expansive microcapsules sufficiently expansive in high temperature region of 200° C. and higher.
Japanese Patent Laid-Open No. Hei 9-19635 disclosed a production process of heat resistant thermo-expansive microcapsules, wherein 80% or more of an acrylonitrile monomer are employed for producing thermo-expansive microcapsules. Thermo-expansive microcapsules are usually applied to a process wherein the microcapsules are heated near to their maximum expanding temperature. The microcapsules produced with 80% or more of an acrylonitrile monomer have limited heat resistance, and do not exhibit sufficient expanding performance at 200° C. or higher.
WO99/46320 disclosed a production process for thermo-expansive microcapsules with acrylonitrile, N-substituted maleimide, a monomer of which homopolymer has a Tg (glass-transition point) from 50 to 200° C., and unsaturated carboxylic acid. In this method a preferable ratio of the unsaturated carboxylic acid is 5 weight percent or less and a ratio greater than 5 weight percent decreases the expanding performance of the resultant microcapsules.
WO99/43758 disclosed a production process of highly heat resistant microcapsules wherein the functional groups in shell wall material crosslink each other when the microcapsules are expanded with heat.
Although the process can provide highly heat-resistant microcapsules, the shell wall of the expanded microcapsules has properties of thermo-setting resin, i.e., poor elasticity and brittleness like glass, due to the crosslinking of the functional groups in the shell wall material during expanding with heat. For this reason, the microcapsules can only be applied under limited conditions and thus to limited end uses.
Recently, processes for producing foamed and molded products wherein thermo-expansive microcapsules are mixed in rubber or resin and expanded with heat in molding the mixture have been proposed. Those processes are advantageous for introducing discrete and uniform air bubbles in molded products easily, though it was difficult in the foaming with foaming chemicals.
For example, Japanese Patent Laid-Open No. Sho 59-1541 disclosed a processes for producing expansive rubber composition containing thermoplastic resin hollow particles in which a blowing agent is encapsulated; Japanese Patent Laid-Open No. Sho 59-138420 disclosed a process for producing expanded products by blending an expanding agent similar to microcapsules in rubber or plastics; and Japanese Patent Laid-Open No. Hei 10-152575 disclosed a process for producing foamed and molded products by blending thermo-expansive microcapsules in thermoplastic resin and processing the blend in extrusion or injection-molding. However, the foamed and molded products of expansive rubber or thermoplastic resin produced in accordance with the processes described in those patents shrink after the expansion of blended microcapsules and thus their weight is not satisfactorily reduced.
Japanese Patent Laid-Open No.2002-226620 disclosed a production process of light-weight molded products containing thermo-expansive microspheres of which polymeric shell wall contains 80 weight percent or more of a nitrile monomer. Even with the process, it is difficult to provide sufficiently expanded light-weight molded products when the products are processed at a temperature higher than the maximum expanding temperature of the thermo-expansive microspheres.
Recently there is an increasing demand for the development of more heat-resistant thermo-expansive microcapsules, which are applicable in various fields. In a process for producing highly foamed and molded products by blending thermo-expansive microcapsules with resin, thermo-expansive microcapsules having a maximum expanding temperature higher than the heating temperature of the resin is preferable for expanding the blend with heat to introduce discrete air bubbles in the resin.
Highly foamed molded products can be produced with the conventional thermo-expansive microcapsules in a process wherein thermoplastic resin blended with thermo-expansive microcapsules is heated at comparatively low temperature, 80 to 160° C. to form the blend. On the contrary, sufficiently foamed product cannot be produced with conventional thermo-expansive microcapsules in a process wherein thermoplastic resin, rubber, or thermoplastic elastomer is knead with microcapsules and processed at 150° C. or higher temperature, because of insufficient heat resistance of those conventional thermo-expansive microcapsules.
The inventors of the present invention have found through their investigation that thermo-expansive microcapsules having superior heat resistance can be produced by forming the shell wall of the capsules with polymer comprising a nitrile monomer and a monomer having a carboxyl group in its molecule.
Although the thermo-expansive microcapsules of which shell wall comprises with a nitrile monomer and a monomer having a carboxyl group in its molecule as major component are heat-resistant, they sometimes fail to expand enough depending on processing conditions, such as kneading into resins, etc.
With further investigation, the inventors found a production process for thermo-expansive microcapsules of the present invention, wherein a nitrile monomer, a monomer having a carboxyl group in its molecule, a monomer having an amide group in its molecule, and a monomer having a cyclic structure in its side chain are employed to produce thermo-expansive microcapsules having superior heat and solvent resistance and excellent expanding performance in broad temperature range in high temperature region, and applicable in foaming and molding thermoplastic resin and thermo-setting resin to be molded at 200° C. or higher temperature.
The microcapsules mentioned above rarely change their color in resins owing to their superior heat resistance to that of conventional thermo-expansive microcapsules, and contribute to producing foamed and molded products retaining high degree of whiteness even after processing at 200° C.